Method of making dry electrodes

ABSTRACT

Dry electrodes useful for attachment to individuals made by mixing silver powder with a cement, diluting the mixture with a suitable solvent for the cement and applying it in a thin layer to a surface from which it can be removed as a film when dry; cutting or shaping the dried film to form the size electrodes desired; placing a wire lead on the cut film piece; applying a small amount of solvent to the film surface having the wire thereon; and pressing a second piece of the dried film thereto to form the electrode of the invention.

United States Patent Filed: Oct. 7, 1970 Appl. No.: 78,704

Related U.S. Application Data Division of Ser. No. 760,389, Aug., 1968,Pat. No. 3,547,105.

U.S. Cl. ..29/25.l4, 29/2518, 29/630 A,

29/482, 156/308, 156/264 Int. Cl. ..H01j 9/00 Field ofSearch..29/25.l,25.1l, 25.13, 25.14,

Low et a1. May 16, 1972 4] METHOD OF MAKING DRY 1 References CitedELECTRODES UNlTED STATES PATENTS [72] Inventors: George M. Low, DeputyAdministrator of 1,981,141 11/1934 Caprio ..156/307 the NationalAeronautics and Space Ad- 2,448,173 8/1948 Cowan ....156/308 Xministration with respect to an invention gerstener et f; F k B. R 1403w tK k raun fi 'gjl es emc 3,462,342 8/1969 Cooper et al. ....l56/307 x3,565,059 2/1971 Hauser et a1. ..29/25. 17 X Primary Examiner.lohn F.Campbell Assistant Examiner-Richard Bernard Lazarus Attorney.lohn R.Manning, Monte F. Mott and Paul F. Mc- Caul [57] ABSTRACT Dry electrodesuseful for attachment to individuals made by mixing silver powder with acement, diluting the mixture with a suitable solvent for the cement andapplying it in a thin layer to a surface from which it can be removed asa film when dry; cutting or shaping the dried film to form the sizeelectrodes desired; placing a wire lead on the cut film piece; applyinga small amount of solvent to the film surface having the wire thereon;and pressing a second piece of the dried film thereto to form theelectrode of the invention.

5 Claims, 6 Drawing Figures PATENTEDMAY 16 m2 FIG. la

INVENTOR FRANK B. RAMME ATTORNEYS CROSS-REFERENCES TO RELATEDAPPLICATIONS This application is a division of application Ser. Number760,389, filed Aug. 29, 1968 now U.S. Pat. No. 3,547,105.

ORIGIN OF INVENTION The invention described herein was made in theperformance of work under a NASA contract and is subject to theprovisions of Section 305 of the National Aeronautics and Space Act of1958, Public Law 83-568 (72 Stat. 435; 42 USC 2457).

BACKGROUND OF THE INVENTION l. Field of the Invention The invention isin the field of dry electrodes. More particularly the invention is inthe field of dry electrodes for use in physiological monitoring ofindividuals.

2. Description of the Prior Art Prior to the herein invention there havebeen developed dry electrodes for application to individuals. Theelectrodes were for physiological monitoring, such as forelectrocardiograms and electroencephalograms. This work is described inNASA Technical Note D-34 l 4, May 1966, Dry Electrodes for PhysiologicalMonitoring, by Charles W. Patten, Frank B. Ramme and James A. Roman.This prior method involved forming a solution of conductive electrodematerial to be sprayed on to the individual. The solution was comprisedof powdered silver in a cement, such as Duco cement made by the DuPontCompany, which is cellulose nitrate dissolved in a suitable solvent. Theviscosity was controlled by adding more solvent for the cement such asacetone, so that the mixture could be readily sprayed through anatomizer. Upon evaporation of the solvent from the sprayed mixture, adry conductive electrode of the silver disposed in the cement wouldreside on the person sprayed.

In the practice of this prior technique, a small amount of conventionalelectrode jelly was wiped on the area of the skin to which the electrodewas to be applied. The lead wire utilized was Teflon insulated copperwire that had a pin connector on one end for eventual connection to thereadout instrument, while the other end of the wire was stripped with asmall epoxy ball fastened to the end tip. The stripped end of the wirewith the ball thereon was placed against the nozzle of the spray gunutilized. The nozzle was then placed against the skin and centered overthe jelly coat. A coating of the conductive cement mixture was thensprayed onto the skin to fonn about a half-dollar size electrode andcapture the lead wire in the cement. The same spray gun was thenutilized by a valve control to spray air only against the driedelectrode to rapidly dry it.

Care had to be taken to withdraw the gun about 2 seconds after the startof the air dry to prevent the gun from adhering to the sprayed area onthe skin. The aforegoing steps were repeated for each electrode to beapplied to the individual. Normally three different electrodes areapplied, for example, in taking electrocardiograms. Additionally, forbest results, the wire should not contact the skin. Thus, a thin coat ofthe conductive mixture is preferably sprayed and dried before the wireis pressed onto the skin, then the spraying continued to place a furthercoat over it and the wire. As can be seen, the aforegoing is a fairlytime consuming process with certain critical steps. Additionally, theequipment to spray the conductive mixture is relatively expensive and issubject to fouling or clogging and the like. Thus, though the concept offorming electrodes in situ on the body would appear to be expeditiousand advantageous, there are several drawbacks.

Thus, an object of this invention is to provide a dry electrode andmethod of making same which is prefabricated prior to application tosubject.

Another object of this invention is to provide a dry electrode andmethod and making same which is readily and easily applicable to asubject.

SUMMARY OF THE INVENTION The herein invention comprises mixing asuitable cement, such as a cellulose nitrate type cement, with silverpowder in about a 50 50 weight ratio, together with enough solvent toprovide a fluid mixture to be sprayed. The solvent preferably used is anorganic material such as acetone. The mixture is then sprayed on a sheetof inert material such as polyethylene to a desired thickness. Afterdrying, the conductive cement is detached from the plastic sheet andplaced upon a sheet of material such as rubber so that discs can bepunched from the cement. Utilizing a conventional punch, discs of anychosen diameter can then be provided from the dried sheet of conductivecement. To form the completed electrode a wire having a stripped end isplaced on one disc resting on a sheet of inert plastic, a drop or two ofacetone or other suitable solvent for the cement is then placed on thedisc, and a second disc immediately then pressed over the wire to theunderlying first disc to form a sandwich about the wire. The product isagain allowed to dry. The final dry electrode thus has two discssandwiched about the lead wire. The two discs, of course, are heldtogether by the adhesive action due to the action of the solvent and canthen be readily applied to the subject by wiping a desired section ofthe subject's body with acetone and pressing the dried disc on thatarea, leaving it there until the acetone evaporates. It is believed theinvention will be better understood from the following detaileddescription and drawings in which:

FIGS. la-ld schematically depict the steps of forming the dry electrodeof this invention;

FIG. 2 is a top view of a dry electrode of the invention;

FIG. 3 is a sectional view taken along line 33 of HO. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning now to the drawingthere is seen in FIG. la, a sheet of plastic 11 with a nozzle 13 from anatomizer or like assembly disposed adjacent thereto. The nozzle 13sprays a conductive cement on the plastic 1 1. The conductive cement canbe of a variety of materials. It has been found that the metal portionof the cement should be preferably silver powder. The metal should beconductive. However, it should not readily oxidize since this interfereswith the electrical properties. For this reason, silver is preferableover a material such as copper. Another metal that could be used isplatinum. However, it is considerably more expensive.

The metal powder is initially mixed with a cement material which shouldbe fast drying and adherent to the skin. It is apparent that the basiccement material should also be inert to the body so that it will notirritate or cause discomfort. Additionally, it is important that afterthe cement has dried it can be readily redissolved at its surface sothat one can easily and subsequently attach the formed electrode to thebody. Thus, various organic cements such as cellulose nitrate, celluloseacetate, polyvinyl acetate and various acrylic resin adhesives arecontemplated. These materials all possess the foregoing adhesivequalities and can be readily and easily dissolved in an organic solventwhich will evaporate readily.

The organic solvent which serves the purpose of decreasing the viscosityof the cement so that it can be easily sprayed from the atomizer head13, should also, of course, be a material that will be tolerated by thebody as well as rapidly evaporating. Acetone is found to be preferableas a solvent material to serve the aforegoing purposes. However, otherorganic materials such as methyl ethyl ketone, butyl acetate, amylacetate, and the like can be utilized in addition to or in place ofacetone.

To form the initial mixture, silver or other metal used is added to thecement on the weight ratio of to percent of silver to the amount ofcement present. The amount of acetone or other suitable solvent added tothe mixture depends solely upon the spray apparatus utilized. Onlyenough acetone should be used so that an adequate spray can be obtained.Too fluid a material is not readily handled, and the thickness cannot beeasily controlled when applied to the plastic sheet 11. The plasticsheet 11 should be of a material that is inert to the cement so it willnot adhere to it. For example, a typical plastic would be polyethyleneplastic.

To control the thickness of the conductive cement applied to the plasticsheet 11, one can maintain the surface area of a portion of the sheet ata given size, and vary the amount of material applied to that surface.Thus, by applying twice the quantity of the conductive cement to a givensurface area, one will double the thickness of the final product. Thethickness is not critical. The cement product can vary from about 7 milsto one that is several times as thick.

After the conductive cement has been sprayed onto the surface 11, it isallowed to dry. This can be aided by means of a fan to circulate airover the surface, aiding in the evaporation of the solvent used. Uponcompletion of the drying, the conductive cement 15 is then peeled, as inFIG. lb, from the base 1 1. This produces a continuous sheet of adesired thickness of conductive cement.

The dried sheet of conductive cement 15, after it is peeled as shown inFIG. lb, is then placed as seen in FIG. 10, on a mat 17 of hard rubberor the like. A punch 19 is then used to cut disc elements from the sheet15 as seen at the remaining holes 21. The discs removed are preferablythe size of a half-dollar but can obviously be of any suitable diameter.

To form the complete electrode element, two discs, as shown in FIG. Id,are utilized. A first disc 23 has a drop or two of solvent applied toits surface. The lead wire 25 is then laid upon the disc as shown. Asecond disc 27 is then placed on top of the first disc 23, sandwichingthe wire therebetween, and pressed upon the first disc 23. The solventrapidly evaporates, leaving a solid electrode member. The last operationcan be performed while the bottom disc 23 is once again resting on asheet of plastic material similar to or the same as the sheet used inthe initial spraying step shown in FIG. la.

FIGS. 2 and 3 combined show a completed unit. The insulated lead wire 25preferably is exposed along the portion 29 that extends between the twodiscs 23 and 27, and is silver plated thereat to provide a non-oxidativesurface in this region, as well as improve the conductivity from theconductive cement to the wire. The wire 25 is then capable of beingconnected to a suitable plug which can also receive the other electrodesutilized. The plug in turn can be attached to a line fed into anelectrocardiogram apparatus or the like.

To use the completed dry electrode as shown in FIGS. 2 and 3, a portionof the body to which the electrode is to be applied is wiped or dabbedwith the solvent used for the cement, such as, for example, acetone. Theelectrode is also wetted with solvent, then immediately pressed upon thewet area of the body and held for a second or two until the electrodehas started to adhere. It has been found that the cement is notconductive when wet with solvent. Thus, one of the advantages of theherein invention as compared to the prior art spraying device is thatthe electrode when applied is basically dry. This small layer of solventis absorbed into the skin as well as the dry electrode at an extremelyrapid rate. Thus, the electrode is ready for use much sooner than in thespray-on device which takes longer to dry.

In order to remove the electrode from the body, one can simply peel itoff in the manner of an adhesive bandage, or alternatively, where thearea might be covered with hair or the like, the entire electrode can bedissolved when wiped with the liquid solvent. The invention will befully understood from the following example.

EXAMPLE To form a conductive cement mixture capable of being sprayed, 43grams of Duco household cement, manufactured by E. I. DuPont de Nemours8L Company, which is a cellulose nitrate cement, containing acetone andbutyl acetate as solvents therefor to ether with a high boiling esterlasticizer, was mixed with 4 grams of silver powder and 10 millilitersof acetone. The mixture was sprayed onto a sheet of polyethylene plasticover a square area of 6X6 inches. This produced a thickness ofapproximately 7 mils over that area. The conductive layer was thenpermitted to dry into a pliable non-tacky solid surface on the plasticsheet. The conductive layer was then peeled off of the underlyingplastic and laid upon a sheet of neoprene rubber. Discs of 34-inchdiameter were then cut with a punch from the sheet of conductive cement.To form the completed electrode, a cut disc was then laid upon a sheetof polyethylene. Two drops of acetone were placed on the top exposedsurface of the disc and a lead wire, which was silver plated in theregion of contact with the disc, was laid upon it. Immediately, a seconddisc was then placed on top of the first one sandwiching the wiretherebetween. Pressure was applied to the two discs for a second or twoto allow the acetone solvent to thoroughly blend the two discs togetherabout the wire. The electrode together with its lead wire was then readyto be attached to a suitable plug for eventual connection to a readoutapparatus.

It should be apparent that the invention can be practiced with obviousmodifications to the process described. For example, the initial mixtureof conductive cement can be applied by methods other than spraying. Itcan be rolled onto a base or applied through a dipping. In other words,various techniques can be used to obtain a dry film of the conductivecement to be cut into discs.

Additionally, the conductive cement does not have to be removed from thebase prior to punching the discs. It is just believed to facilitate theseparation of the cement from the plastic sheet if it is accomplishedbefore the discs are punched.

Iclaim: l. A method of forming dry electrodes comprising: mixing acement and a conductive metal powder with sufficient solvent for saidcement to form a flowable composition capable of being readily appliedto a surface,

applying the mixture on an inert surface to form a conductive cementsheet of desired thickness,

drying said sheet,

cutting pieces from said sheet to use in forming electrodes,

placing solvent and an electrical lead wire between two of said piecesto form said electrode,

and allowing said formed electrode to dry.

2. The method of claim 1 wherein said cement and metal powder is mixedwith sufficient solvent to permit spraying of the resulting mixture, and

spraying said mixture on said inert surface to said desired thickness.

3. The method of claim 1 further comprising:

peeling said dried sheet from said inert surface prior to cutting saidpieces therefrom.

4. The method of claim 1 wherein said cutting of pieces from said sheetcomprises punching circular discs.

5. The method of claim 1 wherein said metal powder is silver and furthercomprising:

silver plating the portion of said lead wire that is placed between saiddiscs.

i 4 l O t

2. The method of claim 1 wherein said cement and metal powder is mixedwith sufficient solvent to permit spraying of the resulting mixture, andspraying said mixture on said inert surface to said desired thickness.3. The method of claim 1 further comprising: peeling said dried sheetfrom said inert surface prior to cutting said pieces therefrom.
 4. Themethod of claim 1 wherein said cutting of pieces from said sheetcomprises punching circular discs.
 5. The method of claim 1 wherein saidmetal powder is silver and further comprising: silver plating theportion of said lead wire that is placed between said discs.